1. Field of the Invention
The present invention relates to an automatic frequency control circuit (hereinunder referred to as AFC) for converging input signals on any desired frequency of intermediate frequencies, and more particularly to an AFC for a receiver of phase shift keying (hereinunder referred to as PSK) modulated signals.
2. Description of the Related Art
Recently, a PSK modulation has been widely used as a modulation of radio signals.
In particular, a .pi./4 shift QPSK (hereinunder referred to as QPSK) modulation for shifting round a symbol phase every 45.degree. has attracted attention. The QPSK modulation modulates the frequency of the carrier wave with transmit signals, and the frequency of the received signals also varies every moment. Thus, it is not possible to adopt an AFC which generates any desired intermediate frequency signal (hereinafter referred to as IF signal) from the received signals and the local oscillation signals.
In order to overcome the above-described problem, there is proposed the AFC disclosed in Japanese Patent application laid-open Heisei 5-22723 (hereinafter referred to as Reference 1) of Sept. 3rd, 1993, for example.
The AFC described in Reference 1 detects the frequency of the IF signal as well as the frequency shift of the IF signal due to the QPSK modulation, and to produce a compensation value. Further, the frequency of the IF signal is compensated by using the compensation value, and the frequency deviation caused by the QPSK modulation is eliminated.
Although the AFC explained in Reference 1 can obtain a desired IF signal only when the frequency of the IF signal are shifted due to the .pi./4 QPSK modulation, it is not possible to cope with the frequency deviation of the reference oscillation circuit for the intermediate frequency signal.